1. Field of the Invention
This invention relates to a drilling tool, and in particular to a rotary drilling tool intended for use in the drilling of boreholes, for example for subsequent use in the extraction of oil or natural gas.
2. Description of the Related Art
It is sometimes desirable to incorporate, in a borehole, regions of increased diameter. Obviously, if a relatively large diameter portion is to be drilled beneath a smaller diameter portion, the bit capable of drilling the large diameter portion has to be able to fit through the smaller diameter portion. One type of drill it is used in such applications includes a number of moving parts to allow the effective drilling diameter of the bit to be increased. Another type of drill bit used in this sort of application is a bi-centre drill bit. Such a bit is designed so as to permit it to be driven about two different axes of rotation. Rotation or support of the bit about a first, central axis results in the bit drilling or being capable of being passed through a relatively small diameter hole, rotation of the bit about a second axis eccentric to the main body of the bit resulting in a larger diameter being drilled. A pilot region of the drill bit is centred upon the second, eccentric axis of rotation and serves to drill a pilot hole, engagement of parts of the bit with the wall of the pilot hole serving to stabilise the bit.
For maximum stability, it is desirable for the pilot region to be of large diameter. However, increasing the size of the pilot region results in the two axes being fairly close to one another, reducing the maximum diameter of hole that can be drilled for a given size of tool.
EP 0058061 describes a bi-centre drill bit of the general type outlined hereinbefore. Similar arrangements are shown in GB 2351513, U.S. Pat. No. 6,394,200, EP 1085167, EP 1039095 and US 2004/0099448.
In one aspect, the invention provides a drilling tool in which the disadvantages set out hereinbefore are overcome or are of reduced effect.